In eukaryotic cells, the critical steps for the regulation of cell proliferation apparently reside in the G1 phase of the cell cycle. Under conditions restrictive for growth, a number of cell strains and untransformed cell lines tend to arrest in a state of quiescence in G1, this resting stage often being referred to as G0. A number of temperature-sensitive (ts) mutant cell lines that arrest at the nonpermissive temperature, in a specific phase of the cell cycle, have been isolated. Two such ts mutant cell lines (AF8 and ts13, both derived from BHK cells) specifically arrest at the nonpermissive temperature in G1. When they are made quiescent by serum restriction and subsequently stimulated with 10% serum, these G1 ts mutants enter S phase at 34 degrees but fail to do so at 40 degrees. Similarly, polyoma infection of resting AF8 or ts13 cells causes these cells to enter S phase at 34 degrees but not at 40 degrees. However, infection with Adenovirus 2 causes these mutants to enter S phase, even at the nonpermissive temperature of 40 degrees. We propose a combination of G1 ts mutants and virus infection to identify proteins that regulate the cell cycle flow in critical G1 period. For this purpose, we shall use gel electrophoresis of immune complexes, identification of early viral mRNAs and microinjection into cells of DNA fragments produced by restriction endonucleases.